In recent years, a higher level of properties is being demanded of semiconductors, and their environments of use are becoming increasingly severe. Therefore, although Si was used in the past as a material for semiconductor elements (referred to as Si semiconductor elements), SiC, GaAs, GaN, and the like are now being used. Below, these semiconductor elements will be referred to as SiC semiconductor elements, GaAs semiconductor elements, and Ga Semiconductor elements, respectively. Because semiconductor elements made of SiC, GaAs, and GaN have superior properties such as excellent pressure resistance, an increased operating temperature, and an enlarged band gap, they are being applied to power transistors and optical devices such as LEDs. These semiconductor elements, which are referred to as next-generation semiconductors, can operate at a high temperature, and solder joints used therein may reach 250-280° C. Accordingly, these next-generation semiconductors require a high-temperature solder.
In general, a semiconductor element is connected to a heat sink such as a metal core or a ceramic plate for heat dissipation. High-temperature solders are also used for such applications.
Several high-temperature solders are already known. An example of such conventional high-temperature lead-free solders is an Au-20Sn solder alloy, which has a eutectic Au—Sn alloy composition. Because an Au-20Sn solder alloy has a eutectic temperature of 280° C., it can be used at temperatures from at least 250° C. to less than 280° C. However, it is an extremely expensive material.
Examples of less expensive high-temperature lead-free solder alloys include Sn—Sb based solder alloys, Bi based solder alloys, Zn based solder alloys, and Ag-containing sintered alloys. Among these, Sn—Sb based solder alloys are superior to Bi based or Zn based solder alloys and Ag-containing sintered alloys from the standpoints of thermal conductivity, corrosion resistance, and bonding strength.
Patent Documents 1-3 disclose Sn—Sb—Ag—Cu solder alloys in which Ag and Cu are added to a Sn—Sb solder alloy as high-temperature solder alloys which can be used in a temperature range of 250-280° C.
Namely, Patent Document 1 discloses a Sn—Sb—Ag—Cu solder alloy having a solidus temperature exceeding 300° C. which is achieved by focusing on the ratio of Sn and Sb.
Similar to Patent Document 1, Patent Document 2 discloses a Sn—Sb—Ag—Cu solder alloy having a solidus temperature exceeding 300° C.